In a marine diesel cross-head engine, the cylinder liner and the crankcase are lubricated separately using a cylinder oil and a system oil respectively. The cylinder oil, often referred to as a marine diesel lubricant (or MDCL), lubricates the inner walls of the engine cylinder and the piston ring pack, and controls corrosive and mechanical wear.
Such engines are usually fuelled by heavy fuel oil or marine distillate fuel. These fuels have a high sulphur and heavy metal content, as well as being of high viscosity and being difficult to handle. For example, a heavy fuel oil may have sulphur levels ranging from 50 ppm to more than 4.0% by mass. For engines operating with these fuels, the MDCL has to be designed to provide base to neutralise the acids produced as a result of combustion of the sulphur-containing fuel. Typical MDCL's may have a total base number of 70-100 mg KOH/g (ASTM D 2896-98).
More recently, efforts are being made to reduce fuel sulphur levels in marine fuels in order to reduce the adverse environmental impact of large marine engines.
This invention is concerned with using low sulphur fuels such as liquid natural gas (LNG) as the fuel. Since LNG predominantly consists of methane, with the balance made up of other hydrocarbons, the MDCL does not require excess base to neutralise acids. It is, however, still required to provide wear protection and cleanliness to the cylinder liner and piston area of the engine. Low sulphur fuels generally have a sulphur level of 0.5% or less.
WO 2011/051261-A ('261) generally describes lubricants having a TBN of at least 10 mg KOH/g for improving deposit formation in marine diesel engines. '261 exemplifies formulations of marine cylinder oils for use in marine diesel engines. However, all examples are conducted at TBN's in excess of 20 and the specification makes no mention of LNG-fuelled engines. '261 states that its best examples are Examples 5 and 6, where the lubricant comprises a low BN Ca sulphonate and a high BN Ca phenate.
A problem in the art is to provide MDCL's for use in a LNG- and similarly fuelled marine cross-head engine where the MDCL has a low base content, but yet is still capable of providing wear protection and cleanliness properties.